I wouldn't call the Lunar XPrize an unsolved problem. We have already landed manned vessels on the moon, and have landed and operated rovers on other celestial

I wouldn't call the Lunar XPrize an unsolved problem. We have already landed manned vessels on the moon, and have landed and operated rovers on other celestial bodies. The issue is more that we do not care enough to land a rover on the moon. Of course, if you interpret the challenge as doing it withing a budget of $20,000,000, then it becomes a much more interesting challenge.